The invention relates to injecting fluid into an electrical cable.
High voltage electrical connectors are used in forming circuits connecting electrical equipment, such as transformers and circuit breakers, to distribution systems and the like through high voltage cables typically having 15 to 35 kV of electric potential. The connectors are configured such that at least one of the cables may be disconnected easily from one of the connectors to create a break in the circuit.
The connectors must be handled by service personnel while powered, which means there is some risk that the connector and disconnecting cable may experience corona discharges and other electrical discharges. For this reason, the connectors include many safety features to minimize the risk of injury and the chance of structural damage to the connector and to other nearby equipment.
A conventional high voltage electrical connector, often referred to as an elbow connector, includes a cable connector assembly within the body of the elbow connector for interconnecting or electrically coupling one cable within the high voltage electrical connector to a mating electrical contact structure of an associated, mating bushing. The mating bushing is, in turn, electrically connected or coupled to a transformer or other piece of electrical equipment. The cable connector assembly is surrounded by an insulative dielectric material, except for openings providing access to the internal connector assembly. The insulative dielectric material is surrounded by a conductive shield, which may be in the form of a molded boot. The conductive shield is electrically connected to ground so that any voltage which may form on the surface of the insulative dielectric material or any electrical discharge near the connector is immediately dissipated to ground.
In many instances, it is desirable to have access to the interior of the high voltage electrical connector. For example, it is often desirable to vent gases from the interior of the connector, conduct tests on the interior cable connector assembly, or take measurements from within the connector. Thus, a high voltage electrical connector may include an access hole extending from the outside of the connector and through the insulative material to expose the internal cable connector assembly.
The cable connected to the connector typically includes a continuous, cylindrical insulative sheath surrounding the high voltage conductive interior of the cable. This insulative sheath is surrounded by a grounded conductive sheath of metallic wires located on the exterior of the cable. The conductive sheath keeps the cable at ground potential, ties all neutrals together, and provides a return path for any fault current that may flow due to cable failure.
A connector access hole may be used to inject an insulative liquid into the connector and the cable extending from the connector to improve the dielectric strength of insulative material within the connector and the cable. This insulative liquid restores damaged insulation to rejuvenate the connector and the cable. Restoring the damaged insulation serves to prevent cable failure that may occur if water or other contaminants enter and deteriorate the insulative sheath of the cable. The insulative liquid may be injected into the connector and forced along the entire length of the cable. After injection, the insulative liquid penetrates the molecular structure of the cable insulation and cures in place. This re-establishes the original dielectric strength of the cable to substantially reduce the potential for cable failure.
Regardless of the reason for requiring access to the interior of the connector, one serious potential problem associated with conventional connectors is that arcing or corona discharges may occur when attempting to use the access hole. This may occur, for example, when insulative fluid is injected into the electrical connector and the attached cable through the access hole.
A conventional high voltage electrical connector includes a projection of insulative material extending from the grounded conductive shield of the connector body. See, eg., U.S. Pat. Nos. 4,946,393 and 5,082,449. The access hole is formed in this projection. Because the insulative projection represents a break in the grounded conductive shield, a separate conductive cap of elastomeric material is configured to fit over the insulative projection and abut against the conductive shield of the body to maintain the integrity of the grounded shield. Typically, an insulating rod attached to the conductive cap extends into the access hole when the cap is in place. To this end, the cap includes a cavity for receiving a head of the insulating rod in an interference fit to attach the two components to each other. When the cap is positioned over the insulative projection, the insulating rod fits within the access hole in an interference fit to provide a dielectric seal.
When the cap and the attached rod are removed from the projection, the dielectric seal is broken and the insulative projection is exposed such that there is a large break in the grounded conductive shield. Capacitive coupling may result in this exposed insulative projection having a high electric potential, especially near the base of the projection, even though the insulative material may have excellent dielectric characteristics. Thus, when the cap is removed from the insulative projection, the surface of the projection may be floating at a voltage higher than ground. This voltage may cause corona discharges.
After the cap and rod have been removed, an injection port may be inserted into the access hole. Corona discharges may occur during this insertion process because the insulative projection is exposed without a ground shield and the dielectric seal has been broken.
The injection port permits a gas or liquid to be injected into or removed from the interior of the connector or cable through the hole formed in the injection port. Conventional injection ports are formed from an insulative material and are sized to fit within the access hole to provide a dielectric seal, such as the seal provided by the insulative rod. Conventional injection ports do not include a grounded shield.
Before or after insertion of a conventional injection port into the access hole, a hose or similar item is connected to a hose connector on the injection port so that the desired maintenance, fluid injection, or tests may be initiated. Because the injection port is not covered with a grounded shield, the surface of the insulating projection and the hose connector may have a dangerous electric potential. This potential may cause arcing. Furthermore, the opportunity for a high surface voltage due to capacitive coupling is enhanced because the liquid, gas or contaminants within the cable that are removed from the electrical connector or cable may be good conductors.
Conductive gases or liquids exiting from the injection port also may result in electrical arcing directly out of the hole in the injection port, with the arcing originating in the high voltage internal components of the connector. Because the injection port has no conductive shield, dielectric breakdown of the surrounding air may occur, resulting in arcing to the external surface of the electrical connector and/or other external items, and thereby causing damage or injury.
Thus, when a conventional conductive cap is on an insulative projection with the attached insulating rod properly placed in the access hole, the ground shield and dielectric seal are operable and capacitive coupling to the surface of the insulating material does not pose a problem. However, when the cap and rod are removed, the insulative projection of the connector is exposed and may have a dangerous electric potential. Furthermore, when the cap and rod are replaced with an injection port, the entire surface of the injection port may float at some voltage significantly higher than ground, such that a serious risk of electrical discharges exists when attempting to service the electrical connector through the injection port.
In one general aspect, a dead front system for providing fluid access to an electrical connector and cable includes an injection plug, a fluid access system, and a tube connected at a first end to the injection plug and at a second end to the fluid access system. The injection plug, fluid access system, and tube are surrounded by a conductive, grounded surface.
Embodiments may include one or more of the following. For example, the conductive, grounded surface may include a conductive container or sack, such as a metal mesh container or bag, which contains the fluid access system. The container may be flexible or rigid. The conductive, grounded surface also may include conductive surfaces formed on outer surfaces of the injection plug, the tube, and the fluid access system, which surfaces may be in the form of coatings. In general, the conductive surface and conductive container or sack are electrically connected to each other and to system ground.
The conductive outer surface may be removed from a portion of the first end of the tube that is inserted into a channel in the injection plug. The connection between the tube and injection plug may further include a coating between the inserted portion of the first end of the tube and the channel. The coating may be an adhesive.
The dead front system also may include a tapered collar surrounding the tube and a conductive nut that secures the tapered collar and the tube to the injection plug so that the tapered collar forms a seal against the injection plug. The conductive nut may be made of an insulating plastic coated with a conductive layer.
The dead front system""s fluid access system may include a chamber made from an insulative material, a fluid control system, and a housing. The fluid control system controls the flow of fluid between the chamber and the tube, and may include valves and active components, such as a pump. The housing surrounds the fluid control system and is made from an insulative material. The chamber and the housing may include integral ground shields connected to the system ground. The integral ground shields may include conductive coatings.
The dead front system provides considerable advantages. For example, the system does not have exposed voltages on the exterior of any parts during the process of filling a cable with fluid. This significantly reduces the risk of shock or injury to service personnel, or damage to equipment in proximity of the dead front system during use of the system. In addition, there also are advantages obtained when all of the components in the dead front system""s conductive sack are provided with a conductive integral ground shield because the shield drains off surface charges without audible or visible display.
Other features and advantages will be apparent from the following description, including the drawings, and from the claims.